1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus which provides power assist of an electric motor directly to a steering system so as to reduce manual steering effort to be applied by the driver.
2. Description of the Related Art
FIG. 1 of the accompanying drawings diagrammatically shows the general construction of an electric power steering apparatus of the type concerned. The electric power steering apparatus 1 includes an electric motor 10 incorporated in a steering system, and a control unit 20 for controlling power assist supplied from the electric motor 10, so as to reduce the manual steering effort or force required by the driver.
The steering system includes a steering wheel 2 attached to an end of a steering shaft 3. The opposite end of the steering shaft 3 is connected to one end of a connecting shaft 4 via a first universal joint 4a, the other end of the connecting shaft 4 being connected via a second universal joint 4b to a pinion 6 of a rack-and-pinion mechanism 5. The pinion 6 is in mesh with a rack 7 which is a long bar with gear teeth 7a cut into one side. The rack-and-pinion mechanism 5 translates a rotary motion of the pinion 6 into an axial reciprocating motion of the rack 7. Opposite ends of the rack 7 are connected via tie rods 8 to steerable left and right front wheels 9, 9. When the steering wheel 2 is manually turned or rotated in a desired direction, the rack-and-pinion mechanism 5 and the tie rods 8 cause the front wheels 9 to pivot in the same direction to thereby change the direction of movement of a motor vehicle.
In order to reduce the manual steering effort or force required by the driver, the electric motor 10 is disposed in concentric relation to the rack 7 and supplies an assist torque (steering assist torque) to the rack 7 via a ball screw mechanism 11. The ball screw mechanism 11 converts rotational power of the electric motor 10 into an axial thrusting force acting on the rack 7. The ball screw mechanism 11 is generally comprised of a nut 12 connected to a rotor of the electric motor 10, and a threaded screw portion 7b formed along a longitudinal portion of the rack 7. By virtue of the threaded engagement between the nut 12 and the threaded screw portion 7b, a rotational force of the nut 12 is converted into an axial thrusting force of the rack 7. Thus, the assist torque generated by the electric motor 10 is translated into the axial thrusting force of the rack 7 by which manual steering effort required by the driver to turn the steering wheel 2 is reduced.
A steering torque detecting section (steering torque sensor) 18 detects a manual steering torque Ts acting on the pinion 6 and supplies a torque signal Tp indicative of the detected steering torque Ts to the control unit 20. The control unit 20 outputs, on the basis of the torque signal Tp, a motor control signal 20a to control output power (steering assist torque) of the electric motor 10.
FIG. 2 of the accompanying drawings shows in block diagram the general arrangement of a first conventional control unit. The control unit 20A includes a target assist torque determining section 201 and a motor drive section 202. The target assist torque determining section 201 determines a target assist torque on the basis of the torque signal Tp and outputs the determined target assist torque in the form of a target assist torque signal 201a. More specifically, the target assist torque determining section 201 sets the target assist torque to be zero when an absolute value of the steering torque is less than a predetermined dead zone threshold. Conversely, when the absolute value of steering torque is greater than the predetermined dead zone threshold, a target assist torque which is proportional to the steering torque is output from the target assist torque determining section 201. The target assist torque output from the target assist torque determining section 201 is limited below an upper limit even when the steering torque increases excessively.
The motor drive section 202 outputs, on the basis of the target assist torque signal 201a, a motor drive signal 20a to drive the electric motor 10 so that the target assist torque is supplied from the electric motor 10.
FIG. 3 shows in block diagram the general arrangement of a second conventional control unit. The control unit 20B includes a first target assist torque determining section 211, a steering torque differentiating section 212, a second target assist torque determining section 213, an adding section or adder 214, and a motor drive section 202.
The first target assist torque determining section 211 determines a first target assist torque on the basis of the torque signal Tp and outputs the determined first target assist torque in the form of a first target assist torque signal 211a. More specifically, the first target assist torque determining section 211 sets the first target assist torque to be zero when an absolute value of the steering torque is less than a predetermined dead zone threshold. Conversely, when the absolute value of steering torque is greater than the predetermined dead zone threshold and less than a predetermined threshold, a first target assist torque which is proportional, with low gain, to the steering torque is output from the first target assist torque determining section 211. A steering torque greater than the predetermined threshold causes the first target assist torque determining section 211 to output a first target assist torque which is proportional, with high gain, to the steering torque. The first target assist torque output from the first target assist torque determining section 211 is limited below an upper limit even when the steering torque increases excessively.
The steering torque differentiating section 212 determines a variation per unit time of the torque signal Tp and outputs the determined variation in the form of a differential torque signal 212a (Tp.multidot.s in a Laplace transform range).
The second target assist torque determining section 213 determines a second target assist torque on the basis of the differential torque signal 212a and outputs the determined second target assist torque in the form of a second target assist torque signal 213a. The second target assist torque output from the second target assist torque determining section 213 is limited below an upper limit even when the differential torque value becomes excessively large.
The adder 214 adds together a signal 211a corresponding to the first target assist torque and a signal 213a corresponding to the second target assist torque and outputs the result of arithmetic operation (addition) in the form of a target assist torque signal 214a.
FIG. 4 shows in block diagram a third conventional control unit which is arranged to control the steering assist torque on the basis of the steering torque and steering velocity. The control unit 20C includes a first target assist torque determining section 221, a third target assist torque determining section 222, a subtracting section or subtractor 223, and a motor drive section 202.
The first target assist torque determining section 221 determines a first target assist torque on the basis of the torque signal Tp and outputs the determined first target assist torque in the form of a first target assist torque signal 221a. A steering velocity detecting section or sensor 19 such as shown in FIG. 1 detects a steering rotational velocity (hereinafter referred to as "steering velocity") .theta.s and outputs the detected steering velocity in the form of a steering velocity signal d.theta.. The steering velocity signal d.theta. is supplied to the control unit 20C. The third target assist torque determining section 222 outputs, on the basis of the steering velocity signal d.theta., a third target assist torque signal 22a to correct the first target assist torque. The subtractor 223 subtracts the third target assist torque signal 222a from the first target assist torque signal 221a and outputs the result of arithmetic operation (subtraction) in the form of a target assist torque signal 223a.
FIG. 5 shows in block diagram a fourth conventional control unit which is arranged to control the steering assist torque in response to the steering torque and steering velocity. The control unit 20D generally includes a first target assist torque determining section 231, a steering torque differentiating section 232, a second target assist torque determining section 233, a third target assist torque determining section 234, a subtractor 235, an adder 236, and a motor drive section 202.
The first target assist torque determining section 231 determines, on the basis of the torque signal Tp, a first target assist torque corresponding to the steering torque and outputs the determined first target assist torque in the form of a first target assist torque signal 231a. The third target assist torque determining section 234 outputs, on the basis of a steering velocity signal d.theta., a third target assist torque signal 234a to correct the first target assist torque. The subtractor 235 subtracts the third target assist torque signal 234a from the first target assist torque signal 231a and outputs the result of arithmetic operation (subtraction) in the form of a subtraction signal 235a. The subtraction signal 235a is supplied to the adder 236.
The steering torque differentiating section 232 determines a variation per unit time of the torque signal Tp and outputs the determined variation in the form of a differential torque signal 232a. The second target assist torque determining section 233 outputs a second target assist torque signal 233a on the basis of the differential torque signal 232a. The adder 236 adds together the subtraction signal 235 and the second target assist torque signal 233a and outputs the result of arithmetic operation (addition) in the form of a target assist torque signal 236a. The target assist torque signal 236a is supplied to the motor drive section 202.
The above-mentioned conventional control units 20A-20D shown in FIGS. 2-5, respectively, have various problems, as described below.
The conventional control unit 20A shown in FIG. 2 controls operation of the electric motor on the basis of only the steering torque and hence is likely to induce a delay in response under the influence of an inertial force produced by the motor or a friction produced in the speed-reducing mechanism (ball screw mechanism). It is therefore difficult to maintain the response to steering input and the stability of a control system with high degree of compatibility. An attempt to increase the gain of steering power assist to improve the response characteristics would deteriorate the stability of the control system, producing parasitic oscillation of the control system.
In the conventional control unit 20B shown in FIG. 3, the differential steering torque signal is used for the correction of steering power assist to thereby improve the response characteristics of the control system. This control unit 20B has a drawback however that a peak in gain (see the broken lined curve shown in FIG. 8A) or a great delay in phase (see the broken lined curve shown in FIG. 8B) of the control system is produced when the steering torque has a low frequency of about 2 Hz to about 4 Hz. The peak of gain or the phase delay forms a resonance point which will deteriorate the control properties of the control unit 20B. Due to an excessively high sensitivity to steering input at the low frequency range, the steering wheel tends to wobble or otherwise become unstable under the influence of external disturbance coming from road surfaces when the vehicle is running on rough terrains. Another drawback with the control unit 20B shown in FIG. 3 is that when the steering torque is below the dead zone threshold of the first target assist torque determining section 211, the signal 214a related directly to the target assist torque is output on the basis of only the differential value of the steering torque. Such a signal 214a fails to provide a firm or positive steering feeling.
The conventional control units 20C and 20D shown in FIGS. 4 and 5 further include a control process achieved on the basis of the steering velocity d.theta. to improve the steering accuracy. Since the first target assist torque determined in response to the steering torque Tp is corrected by subtracting therefrom the third assist torque corresponding to the steering velocity d.theta., the stability of the control system is improved. However, no improvement is expected so far as the response to steering input is concerned.